I agree with Quatto. I don't think most people are really gonna use their weapons. Maybe you all think that they're nice to have because they give you peace of mind, but seriously, how many of you are actually going to get off your moped and get out your knife when you feel threatened? I'm willing to bet that either (a) the offender was in a car and has driven off, or (b) that a better "defense" would be to ride away as fast as possible.
If someone wants to start shit by yelling at you, just pretend you can't hear them. We really don't need more road rage anymore. If you want some revenge and choose to use spark plugs or paint or whatever, that's fine, I guess. Revenge is so very sweet. I wouldn't do it because anyone who sees me could call in my plates and because I doubt the offender would relate "almost hitting that moped" (which they probably don't even realize they did) with "this giant strip of paint on my car."
I carry a small metal flashlight in my purse -- I call it my "creeper flashlight." I use it when I'm walking back from the overflow parking lot late at night, but it's mostly a peace of mind measure. The light is nice and I feel better having it, but I know that there is no way I would be able to inflict significant damage with it.
If I ever did feel threatened, either walking back from the overflow lot or anywhere on my moped, I would probably pull out the biggest wrench I had on me, the pointiest screwdriver I had on me, or a can of carb cleaner. Once again, though, I doubt I would be able to inflict any damage on anyone with any of those tools. I am worthless at fighting; my first reaction to anyone coming at me is to flee or drop to the ground, curl up in a ball, and hope they lose interest. I would only break out the weaponry if I had absolutely no other choice or way out.